Conventionally, a technique is publicly known by which a treatment (rust-proof treatment) for preventing formation of what is called red rust (Fe2O3) is applied to a mold used in die casting and the like.
Patent Literature 1 discloses a coating composition for metal, which consists primarily of Zn—Al alloy powder and water-soluble chromium compound.
The coating composition disclosed in Patent Literature 1 is applied to the surface of a metal, and then is heated for a predetermined time at a predetermined temperature, thereby changing into a rust-proof film. The rust-proof film prevents the red rust from forming on the metal.
In the case of applying the coating composition disclosed in Patent Literature 1 to a mold used in die casting and the like, the rust-proof film is formed on a surface other than the molding surface of the mold (in particular, the surface of the cooling channel on which the red rust is easy to form) because a film (for example, a carbon film) for accomplishing reduction of mold-release resistance and the like is formed on the molding surface of the mold.
In this case, it is disadvantageous in that a step for forming the rust-proof film on the surface other than the molding surface of the mold must be performed, in addition to a step for forming the film such as the carbon film on the molding surface of the mold.
Moreover, when the mold is put in a heating furnace in order to repair the film such as the carbon film formed on the molding surface of the mold, materials constituting the rust-proof film may be scattered, which may have a negative influence on forming the film on the molding surface of the mold. Therefore, it is disadvantageous in that the rust-proof film must be removed when the film such as the carbon film formed on the molding surface of the mold is repaired.
Thus, a conventional rust-proof treatment on the mold causes an increase in time and cost required for rust-proofing the mold.